Spectral gamma detectors may be used in logging while drilling (LWD) activities to measure gamma rays in a formation. Mickael et. al. (U.S. Pat. No. 7,253,401, B2) placed a gamma ray detector in the outer wall of a drill collar to minimize the collar attenuation effects. However, placing the gamma ray detector in the outer wall of the drill collar complicates tool design and makes the tool more expensive. In addition, the gamma ray detector may be more susceptible to wear and less accessible for maintenance.
The gamma ray detector has been placed in a sonde in the bore of the collar. Attempts have been made to use a sonde for spectral gamma ray measurements but these attempts have failed because of collar attenuation effects. Moreover, a gamma ray detector placed in a sonde cannot provide azimuthal measurements without the use of a shield, which reduces gamma ray detector counts.